Spermatogenesis proceeds from proliferation of spermatogonial stem cells, through the unique process of meiosis in spermatocytes, to end with the dramatic morphogenesis of haploid spermatids into spermatozoa. This project examines gene products responsible for the unique structural and functional features of spermatozoa, with the goal of determining the intrinsic and extrinsic mechanisms which regulate expression of genes at different phases of the developmental process of spermatogenesis. For example, the fibrous sheath is a cytoskeletal component of the sperm flagellum assembled late in spermatogenesis. A 78kd fibrous sheath protein is synthesized during meiosis, while a 67kd protein appears in spermatids. These proteins share biochemical and functional properties with intermediate filament proteins. Monoclonal antibody and cDNA sequencing studies suggest these proteins are products of unique intermediate filament genes. Also being studied is a gene for a glyceraldehyde 3-phosphate dehydrogenase enzyme (G3PD) of spermatogenic cells. Northern blot data indicate that this gene is expressed only in spermatogenic cells, with the transcript being present in low amounts in spermatocytes and abundant in spermatids. Although not required by ,spermatids, this G3PD appears to accommodate unique metabolic needs of spermatozoa. These and other recent studies indicate that spermatogenic cell-specific proteins are often variants of known somatic cell proteins and the products of genes expressed only in germ cells. This suggests that unique structural and functional features of spermatozoa are the result of germ cell-specific gene expression.